False positive `file_requires_admin` on Windows

[dimitriepirghie]

On Windows OS, module pyupdater.client.updates. file_requires_admin(file_path) function gives false positive, because open(file_path, "a+") on any process image file while it's being running always raises PermissionError exception.

Propose to use this check instead: os.access(file_path, os.W_OK)

[JMSwag]

@dimitriepirghie SGTM

[pm990320]

Hi I'm also seeing this issue, would it be possible to release the fix?